Quis custodiet ipsos custodes? is a Latin phrase found in the work of the Roman poet Juvenal from his Satires (Satire VI, lines 347–348). It is literally translated as "Who will guard the guards themselves?", though it is also known by variant translations, such as "Who watches the watchers?" and "Who will watch the watchmen?".

The original context deals with the problem of ensuring marital fidelity, though the phrase is now commonly used more generally to refer to the problem of controlling the actions of persons in positions of power, an issue discussed by Plato in the Republic. It is not clear whether the phrase was written by Juvenal, or whether the passage in which it appears was interpolated into his works.

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The phrase, as it is normally quoted in Latin, comes from the Satires of Juvenal, the 1st/2nd century Romansatirist. Although in its modern usage the phrase has universal, timeless applications to concepts such as tyrannicalgovernments, uncontrollably oppressive dictatorships, and police or judicial corruption and overreach, in context within Juvenal's poem it refers to the impossibility of enforcing moral behaviour on women when the enforcers (custodes) are corruptible (Satire 6, 346–348):

I hear always the admonishment of my friends:
"Bolt her in, constrain her!" But who will guardthe guardians? The wife plans ahead and begins with them.

Modern editors regard these three lines as an interpolation inserted into the text. In 1899 an undergraduate student at Oxford, E. O. Winstedt, discovered a manuscript (now known as O, for Oxoniensis) containing 34 lines which some believe to have been omitted from other texts of Juvenal's poem.[1] The debate on this manuscript is ongoing, but even if the verses are not by Juvenal, it is likely that it preserves the original context of the phrase.[2] If so, the original context is as follows (O 29–33):

... I know
the plan that my friends always advise me to adopt:
"Bolt her in, constrain her!" But who can watchthe watchmen? They keep quiet about the girl's
secrets and get her as their payment; everyone hushes it up.

This phrase is used generally to consider the embodiment of the philosophical question as to how power can be held to account. It is sometimes incorrectly attributed as a direct quotation from Plato's Republic in both popular media and academic contexts.[3] There is no exact parallel in the Republic, but it is used by modern authors to express Socrates' concerns about the guardians, the solution to which is to properly train their souls.

Several 19th century examples of the association with Plato can be found, often dropping "ipsos".[4][5]John Stuart Mill quotes it thus in Considerations on Representative Government (1861), though without reference to Plato. Plato's Republic though was hardly ever referenced by classical Latin authors like Juvenal, and it has been noted that it simply disappeared from literary awareness for a thousand years except for traces in the writings of Cicero and St. Augustine.[6] In the Republic, a putatively perfect society is described by Socrates, the main character in this Socratic dialogue.

Socrates proposed a guardian class to protect that society, and the custodes (watchmen) from the Satires are often interpreted as being parallel to the Platonic guardians (phylakes in Greek). Socrates' answer to the problem is, in essence, that the guardians will be manipulated to guard themselves against themselves via a deception often called the "noble lie" in English.[7]
As Leonid Hurwicz pointed out in his 2007 lecture on accepting the Nobel Memorial Prize in Economic Sciences, one of Socrates' interlocutors in the Republic, Glaucon, even goes so far as to say "it would be absurd that a guardian should need a guard."[8]

The question "Who watches the Watchmen?" frequently appears in Alan Moore and Dave Gibbons' graphic novel Watchmen, but the phrase is seen in its entirety only on the closing page of Issue 11.[9] Moore stated in an interview that the title of the series related directly to this question, although at the time of the interview Moore did not know where the sentence originated.[10]

Science fiction author Robert A. Heinlein chose the original Latin phrase as the motto of the Solar Patrol, as depicted in his 1948 novel Space Cadet.

The internet comedy group LoadingReadyRun made a video parodying the question of 'Who watches the Watchmen?', proposing that the Watchmen watch the city, the 'Neighbourhood Watchmen Watching Organisation' watch the Watchmen, the 'Watching The Neighbourhood Watchmen Watching Organisation Organisation' watch the Neighbourhood Watchmen Watching Organisation, and Geoff watches the 'Watching The Neighbourhood Watchmen Watching Organisation Organisation'. An unnamed person is seen to be watching Geoff.[11]

The independent film The Guards Themselves[12] by Kyle C. Sullivan and Ian Conn takes its title from this phrase. It tells the tale of a group of so-called anarchists who appear to be villains endeavoring to overthrow the government and who are thwarted repeatedly by supposedly heroic vigilantes. However, the government in their city is corrupted by an actually villainous group of five oligarchs, and the vigilantes are primarily out for publicity; it is the anarchists who are the true guards of their city.

An episode of Inspector Morse ("Absolute Conviction") references this quote. Whilst speaking with the prison governess Hilary Stephens in an Oxford college, Morse encounters his former college chaplain. Upon finding out that the head of a prison is off prison premises he asks "quis custodiet ipsos custodes?" Morse finds it amusing since the chaplain says this every time the two meet, only this time it is "remotely apposite".

In Dan Brown's novel, Digital Fortress, the phrase appears engraved in a ring owned by Ensei Tankado, a former NSA employee who disapproved of the NSA's intrusion into the people's private lives. The phrase is aimed at the NSA who check for any information on emails sent over the web that endanger national security. The phrase asks who will keep the NSA in check, as they do others.

In Did I Ever Tell You How Lucky You Are? by Dr. Seuss, the entire town of Hawtch-Hawtch is employed as watchers watching over other watchers leading to the first watcher who is watching the "lazy town bee" so it will work harder. Since the bee wasn't working harder, it was assumed the bee-watcher wasn't watching hard enough and needed to be watched.

In the video game Halo: Combat Evolved Anniversary, the seventh Terminal's description is "Who monitors the Monitor?", and details 343 Guilty Spark's frustration with loneliness. The replacement of "watchmen" with "Monitor" is a reference to Guilty Spark's position of Monitor of Installation 04.[14]

In the Person of Interest television series, if The Machine ever suffered a hard reset it would then ring a public phone and ask the phrase, and whoever answered the call would have administrative access for 24 hours.

In the comic book series Countdown to Final Crisis, Donna Troy asks the question "Who monitors the monitors?" in regards to the 52 monitors from parallel universes and who will keep them in line, following the events of the story.

In Letter VIII of "Meditations on the Tarot: A Journey into Christian Hermeticism",[15] on Justice (Tarot card), the anonymous author (now known to have been Valentin Tomberg) includes the phrase in one of the opening epigraphs as follows: "Quis custodiat custodes? (Who will guard the guards?. . . The fundamental problem of jurisprudence)".

In the video game The Evil Within 2, the character Julian Sykes quotes the Latin phrase to protagonist Sebastian Castellanos in reference to Sykes himself having found an escape route, with Sykes abandoning his mission by the company Mobius that hired him. Sebastian expresses confusion at the phrase when given no explanation.

^Recently J. D. Sosin 2000, "Ausonius' Juvenal and the Winstedt fragment", Classical Philology 95.2: 199–206 has argued for an early date for the poem.

^E.g. Who Are The Watchmen?; T. Besley and J.A. Robinson, "Quis Custodiet Ipsos Custodes? Civilian Control over the Military", Journal of the European Economic Association v. 8, pp. 655–663, 2010; and P. Corning, The Fair Society: The Science of Human Nature and the Pursuit of Social Justice,, University of Chicago Press, p. 146, 2011.

^Plato (2008) [c. 380 BC]. The Republic. Benjamin Jowett, transl; EBook produced by Sue Asscher and David Widger. Project Gutenberg. How then may we devise one of those needful falsehoods of which we lately spoke – just one royal lie which may deceive the rulers, if that be possible, and at any rate the rest of the city?

^Book III, XII, 403E, p. 264 (Greek) and p. 265 (English), in volume I, of Plato, The Republic (ΠΟΛΙΤΕΙΑ), with an English translation by Paul Shorey, London, William Heinemann Ltd.; New York: G. P. Putnam’s sons, 1930, as cited by Leonid Hurwicz," But Who Will Guard the Guardians?", Nobel Prize Lecture, December 8, 2007, Accessed 4-27-2011.